We are trying to develop a complete molecular description of the genes that code for ribosomal RNA in the frog, Xenopus. In one approach, we are locating control regions in the DNA -- places where RNA transcription begins, where DNA replication starts -- and these will then be sequenced to define the nucleotide sequences that interact with polymerases and other control proteins. In another line, we are studying the organization of repetitive sequences in the spacers between these genes to gain insight into the mechanism for keeping these multicopy genes homogeneous in sequence. In another line, we are isolating and identifying proteins which bind to these genes. And finally, we hope to develop in vitro systems where transcription of these genes can be regulated accurately. Such research should eventually yield a complete description of the gene structure and the molecular mechanisms whereby their activity is regulated in living cells. BIBLIOGRAPHIC REFERENCES: Bustin, M., Reeder, R. H. and McKnight, S. L., "Immunological Cross Reaction between Calf and Drosophila Histones." J. Biol. Chem. 252, 3099-3101 (1977). Higashinkagawa, T., Wahn, H. and Reeder, R. H. "Isolation of Ribosomal Gene Chromatin." Dev. Biol. 55, 375-386 (1977).